It is well known that as a petroleum resource, e.g., a crude oil or petroleum residuum is distilled to higher cut point, the amount recovered as distillate naturally increases. However, as the cut point increases, the concentration of metallic contaminants in the distillate also tends to increase. Metal-containing compounds, including porphyrin or porphyrin-like complexes, are abundant in heavy petroleum distillates. These organo-metallic compounds can be volatized, thus contaminating the distillate fractions. For example, petroleum distillates such as gas oils for use as feed to a catalytic cracker normally may contain several ppm of metals. However, if deeper incremental distillation cuts are taken and included in the gas oil, then the metals content of such deeper incremental cuts can be much higher. For example, such deeper incremental cuts may reach 50-100 ppm Vanadium or higher. Consequently, the final distillation cut point (end point) of gas oils intended for use as cat cracker feed is conventionally not higher than about 1050.degree. F.
In petroleum processing operations such as catalytic cracking, the presence of a high concentration of metallic contaminants in the petroleum feed leads to rapid catalyst contamination causing an undesirable increase in hydrogen and coke make, an attendant loss in gasoline yield, a loss in conversion activity and a decrease in catalyst life. The effects of these metallic contaminants on zeolite-containing catalysts are described in detail in U.S. Pat. No. 4,537,676. The metallic contaminants are believed to affect the catalyst by blocking the catalyst pore structure and by irreversibly destroying the zeolite crystallinity. The adverse catalytic effects of nickel and vanadium containing compounds, in particular, are discussed by Cimbalo, Foster and Wachtel in "Oil and Gas Journal," May 15, 1972, pages 112-122 and by Bosquet and Laboural in "Oil and Gas Journal," Apr. 20, 1987, pages 62-68.
The removal of metallic contaminants from heavy petroleum distillates such as atmospheric bottoms, heavy gas oils and vacuum gas oils, is becoming increasingly more important as heavier and more metals-contaminated feedstocks are being refined. As a consequence of significant economic incentives, additional efforts are being directed at upgrading such feeds to more valuable products. For example, a sufficiently inexpensive treat of a heavy petroleum distillate to remove metals therefrom could substantially increase the amount of cat cracker feed available.
In the past, efforts have been directed to the removal of metal contaminants from petroleum distillates by a variety of methods including hydro-treating, deasphalting, and acid extraction.
Hydrotreating technology using CoMo, and/or NiMo catalysts is used for upgrading some feeds for catalytic cracking, but a selective hydrotreating process which is capable of essentially only removing metals without consuming substantial amounts of hydrogen in other reactions has not been available.
U.S. Pat. Nos. 2,926,129 and 3,095,368 describe a method for selectively removing iron, nickel and vanadium from an asphalt-containing petroleum feedstock by deasphalting the oil and subsequently contacting the oil with a mineral acid, such as HCl, to coagulate the metallic compound. The metallic compounds are then separated. This process has the disadvantage of requiring the use of deasphalting, which is an expensive operation, and requiring mineral acids which are highly corrosive.
In a paper presented at a meeting of the ACS Division of Petroleum Chemistry Society (Preprints, Vol. 25, No. 2, pages 293-299, March 1980), Bukowski and Gurdzinska disclosed a method for reducing the adverse catalytic effect of metal contaminants present in the distillate from a atmospheric residuum. The method included heat treating the atmospheric residuum in the presence of cumene hydroperoxide (CHP) for up to six hours at 120.degree. C. This step increased the distillate fraction obtained from the atmospheric residuum feed and decreased the metals content of the distillate which subsequently was used as feed for a catalytic cracking unit. This procedure has the disadvantage that the cost of the large amount (2%) of CHP used is relatively high.
British patent application No. 2,031,011 describes a method for reducing the metals and asphaltene content of a heavy oil by hydrotreating the oil in the presence of a catalyst including a metal component from Group Ib, IIb, IIa, Va, VI, and VIII of the Periodic Table and thereafter deasphalting the oil. Relatively large amounts of hydrogen are required.
Various other patents disclose upgrading a residual oil by initially deasphalting and subsequently demetallizing the deasphalted oil, for example, as variously described in U.S. Pat. Nos. 4,447,313, 2,895,902, 3,227,645, 4,165,274, 4,298,456, 3,511,774 and 3,281,350.
The teachings of the prior art, although proposing possible ways to reduce the metals content in a petroleum distillate, fail to provide a process which is sufficiently effective, practical, inexpensive, and which does not suffer from any of the above mentioned drawbacks.